AC/DC Converter for feeding non-linear high-power loads (e.g. Electrolyzers) with reducing current harmonics

ABSTRACT

A new converter is disclosed, for reducing current harmonics in three phase AC network, during the conversion of AC to DC for power supplying the high-power electrochemical and other non-linear apparatus.

BACKGROUND OF THE INVENTION

Power supply of non-linear loads includes; three major equipments; a Power transformer, a rectifier and an electrolyzer. Each of these three components, generate their specific harmonics and combination of the abovementioned harmonic sources in the electrochemical plant. Therefore this causes serious problems due to huge amount of DC current.

Harmonic is an unwanted phenomenon in electrical industry which causes various difficulties for consumers and power supply networks. Harmonic reduction at generating sources; is the basic and desired target for specialists; especially at high-power electrolyzer industries.

The harmful effects of harmonics can be described in the following categories: Firstly on power transformers, it causes overheating and consequently reduction in production. Secondly has an effect on upstream networks; which results in problems with the AC power supplier. Thirdly it has an effect on electrical equipment installed in plants, such as abnormal operation of motors.

The highest sources of harmonics generation, are the industries which apply AC to DC converters with a non-linear, high-rated power loads, since non-linear loads are also sources of harmonic generation. Various solutions are already applied for harmonic reduction. Some of them such as a harmonic filter, a PWM and a Multi-pulse rectifier are not perfectly applicable for high-power sources, and others such an OLTC are complicated and extremely expensive.

On the other hand survey of former inventions at aforementioned industries; shows that:

1) A harmonic filter has no effect on power transformer's overheating, and also has a probability of dangerous resonance. 2) A Multi-pulse rectifier requires complicated transformers, with high losses and is not suitable for high-power application. 3) Other techniques, which were useful for other applications, were not applicable for non-linear high-power loads.

In such situations and abstinence of techniques resolving the above issues and shortcomings, the present invention will be a remarkable solution. Also since the harmonic distortion of thyristors, still has their costs and problems; this condition led me to find a way for changing the future use of rectifiers in those industries; which suffer from huge amount of harmonics.

SUMMARY OF THE INVENTION

AN AC/DC converter to feed high-power electrochemical loads suffers from harmful effects of harmonics which are generated by rectifiers and an electrolyzer. The subject of the present invention is directed to a new and useful AC/DC converter for reducing the current harmonic. This will improve major problems which are affecting the recent electrochemical plants. Some of the solved problems are as follow:

a) Reduction in harmonics that are passing through the power transformers. This prevents the overheating of the transformers and provides production at a nominal capacity. b) Reduction in the harmonics that are injected to upstream network; solving the need of harmonic filters. c) Electrical power loss reduction, which reduces the production cost.

Brief technical description of the invention describes that any regular AC/DC converter consists of three major equipment; a power transformer, a rectifier and a load. Presented invention is based on combination of three regular converters together, in order to integrate their harmonics at specific conditions.

Harmonics, which are generated by various non-linear loads, are different. So any type of non-linear loads requires its specific solution, which is achieved by various configurations. Configuration specifies the hardware of the transformers, the hardware of the rectifiers and also the software of the control, which are necessary to achieve the desired harmonics.

Therefore the detail of the presented invention is not common for all types of loads. In the other word, presented invention is a customized converter and a tailor-made modular package.

It is known that a thyristor generates more harmonics during current control, therefore the presented new converter eliminates harmful effect of thyristor during current control. In the meantime thyristor-based rectifiers suffer from low power factors, in the present new converter; the power factor will be constant and acceptable, so that no power factor correction will be required.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1; Shows three regular rectifiers; each having its own current control

FIG. 2; Shows a schematic diagram of a “Modular Technique” including two transformers Y & Δ, each having three secondary winding, three rectifiers and three isolated loads.

FIG. 3; Shows a block diagram of the “Modular technique” of the presented invention; where all the DC outputs are galvanicly isolated.

FIG. 4; Shows a diagram for the effect of loads' type to the configuration. Therefore any type of non-linear loads; need its specific solution, which we have applied by a configuration. This configuration determines the hardware of the transformers and the rectifiers.

FIG. 5; Shows a control program which has all the thyristors under control; in order to shape the current curve, and consequently a favorite/desired current harmonics and also maintaining a desired DC current value.

FIG. 6; Shows a block diagram of the current control; including a multi profile programmer, a profile selector and drivers, with the capability of individual and total current control.

FIG. 7; displays a diagram of the calculated data for harmonics passing power transformer in CAME configuration; with a comparison between the existing and the future (new) method.

FIG. 8; Displays a diagram of the calculated data for harmonics injected to the upstream (PPC) in the CAME configuration; with a comparison between the existing and the future (new) method.

FIG. 9; Displays a Diagram of the calculated data for the power factor in the CAME configuration; with a comparison between the existing and the future (new) method.

ABBREVIATIONS

The following list describes the abbreviations used in the specification:

OLTC—On Line Tap Changer PWM—Pulse Width Modulation IHD—Individual Harmonic Distortion THD—Total Harmonic Distortion

PCC—Point of Common Coupling (upstream contact)

CAME—Chlore Alkali Membrane Electrolyzer ELEMENT NUMBERS

The following; lists all the element numbers present in the drawings:

-   1 Multi profile programmer -   2 Profile selector -   3 Total coarse control -   4 Drivers -   5 Main control -   6 Individual fine control -   7 Regular converter system -   8 New converter system -   9 New transformer in Yy connection row -   10 New transformer in Y A connection row -   11 New rectifiers in Yy connection row -   12 New rectifiers in Y A connection row -   13 Common DC current control -   14 Three loads in Yy connection row -   15 Three loads in Y A connection row

DETAILED DESCRIPTION

Each regular AC/DC converter (7, FIG. 1) consists of three major equipment; a power transformer, a thyristor-based rectifier and a load. The current invention is based on a combination of three regular converters together (8, FIG. 1).

The harmonic wave has a vector specification. The IHD and THD will be reduced by summation of harmonic waves with equal frequency, in case of preset conditions and in specific moments. Therefore after combining the three regular converters, there will be three power transformers and three thyristor-based rectifiers for feeding the three non-linear loads. In practice, three power transformers are replaced with one power transformer equipped with three secondary windings.

FIG. 1 shows a regular convertor (8) comprising a power transformer, three thyristor-based rectifiers and three isolated DC non-linear loads. All of the rectifiers' thyristors are controlled by a control program. The control is via conducting the specific thyristor at the specific moment. The control leads to a desired current curve and consequently, a desired current harmonic and amplitude.

Presented invention is not a multi-purpose converter, but if needed could be designed to act as one. Some of the industries which can use this invention are chlor-alkali electrolysis, other chlorine derivatives electrolysis, copper refining, and aluminum refining and electroplating (simply examples).

Modular Technique:

One of the popular methods for reducing the harmonics in the high-power converters is; installation of two transformers side by side, one with a Y connection and the other with a Δ connection. FIG. 2 shows the combination of two Y & Δ rows. The current invention uses this technique and employs two Y & Δ rows for more harmonics reduction.

Finally, presented invention has two Y & Δ transformers (FIGS. 2; 9 and 10 respectively) each having three secondary winding, three bridge rectifiers (FIGS. 2; 11 and 12 respectively) and three loads (FIGS. 2; 14 and 15).

All the rectifiers (11 and 12) are under control of a “DC current control” (FIG. 2; 13). This package can be installed in the plant as a modular set. Any number of these modular sets can be installed in each plant.

Configuration

The current harmonic reduction of the non-linear loads is focused in this invention. Harmonics generated by the various non-linear loads are different, and so any type of the non-linear loads needs its specific solution, which is achieved by the configurations.

The configuration determines the hardware of the transformers (9 and 10), the hardware of the rectifiers (11 and 12) and also the software of the control system (13), which are necessary to achieve the desired harmonics. FIG. 4 shows the relationship between the type of loads (14 and 15), the solution and the configuration.

Basics of the Current Control

The DC current in a non-linear load; controlled by a control unit (13); which is equipped with microcontrollers. This control unit is based on, multi-profile control and applies selected profile to the drivers of each rectifier at any required moment.

The current control block diagram is shown in FIG. 6. The Multi profile programmer (1), generates the profiles. The profile selector (2), selects one of the available profiles according to a coarse control signal received from the total coarse control unit (3). The output signal of the profile selector (2); which we have named selected profile signal, controls the drivers (4). The drivers (4) submit trigger pulses for conducting of intended thyristors (18). The main control unit (5), receives all the input parameters such as V_(ac), I_(ac), V_(dc), I_(dc), desired data and other control signals. The consequence for current control (output of the main control unit); sends the current control signal to the total coarse control unit (3), which controls the currents of all the loads in the modular package/set as a coarse control signal.

The main control unit (5) also sends another signal to an individual fine control unit (6), which controls the current of any load separately as a fine control signal via drivers (4 of FIG. 6) of said modular set.

Actually the current control program/unit has all of the thyristors under control, and is capable of conducting the thyristors at any required moment, in order to achieve the desired current curve and consequently the desired current harmonics. This is exactly the same as what a pianist does; where he/she has all the claviers under his fingers and at any required moment, plays some of them to create a favorite tone. FIG. 5, shows that all the thyristors are under control of the current control program (13).

Calculated Data

The present invention is based on the following three main features:

-   -   Reduction of harmonics passing through power transformers     -   Reduction of harmonics injected to a PCC     -   Power factor correction         As mentioned in the configuration chapter, all the parameters         are dependent on the type of the non-linear load, so here we         present calculated data for a specific load, Chlor-Alkali         Membrane Electrolyzer (CAME).

Summing all the features of the present invention, the new converter is presented for reducing current harmonics in DC power supply to high-power non-linear loads; where THD at the power transformer is reduced. Consequently overheating of the power transformer is prevented. THD at PCC is reduced and will be approximately constant. Therefore extra harmonic injections, to the main upstream network, are prevented. Since any types of non-linear loads have their own specific harmonics, new converter have specific configurations for each type of non-linear load. Configuration specifies/determines both the hardware and the software. The power factor will be about 0.97. Therefore no capacitors are required for power factor correction. Also no harmonic filter is required. The heat loss in the rectifiers will be equal to the existing condition and all of the DC outputs to loads are galvanic isolated.

It is important to point out that none of the following methods are applied in the present invention:

Pulse Width Modulation (PWM), Multi-pulse or multi-phase rectifiers, any self-inductive or capacitor for harmonic reduction.

It should also be noted that the invention is not limited to the best modes explained and exemplary features described here, and can be configured and designed to cover a wide range of power circuits. 

1- A converter system for feeding non-linear high-power loads with reducing current harmonics in three phase AC network; comprising: At least one modular set comprising, at least two power transformers, each having at least three secondary windings, at least three bridge rectifiers and at least three non-linear loads; said system further comprises a DC current control unit that is equipped with multiple microcontrollers; wherein said DC current control unit, applies selected profiles to drivers of each rectifier of said modular set. 2- The converter system of claim 1, wherein said at least two power transformers are a Y & Δ transformers and said bridge rectifiers are thyristor-based rectifiers; and wherein said DC current control unit has all of said thyristors under control and conducts any one of said thyristors at any required time. 3- The convertor system of claim 2, wherein said DC current control unit further comprises: a main control unit; a multi-profile programmer; a profile selector; a total coarse control unit; and individual fine control unit. 4- The convertor system of claim 3, wherein said main control unit, receives all input parameters of V_(ac), I_(ac), V_(dc), I_(dc), desired data and other control signals; and sends a current control signal as an output to said total coarse control unit; wherein said multi-profile programmer generates available profiles for said system and wherein said profile selector, selects one of said available profiles (selected profile), based on said output coarse control signal of said total coarse unit; wherein said selected profile controls said drivers of said modular set, where said drivers submit trigger pulses to any of intended said multiple thyristors; wherein said output coarse control signal controls currents of all of said loads in said modular set. 5- The convertor system of claim 4, wherein said main control unit further sends another signal to said individual fine control unit, wherein its output signal (fine control output signal) controls said currents of any of said loads separately via said drivers. 6- The convertor system of claim 5, wherein said system harmonics are reduced and consequently overheating of said power transformer is prevented. 7- The convertor system of claim 6, wherein in said system none of a Pulse Width Modulation (PWM); Multi-pulse or multi-phase rectifiers; any self-inductive or capacitor for harmonic reduction has been used. 8- A method of reducing current harmonics in three phase AC network, comprising the steps of: A Configuration step wherein harmonics generated by any type of high-power non-linear load in an AC/DC convertor is reduced; wherein said configuration step comprises determination of hardware structure and detail of at least two transformers of said convertor; each having three secondary windings, three bridge rectifiers and three non-linear loads; and configuration of said rectifiers of each of said transformers as well as determination of software program of a control system. 9- The method of claim 8, wherein said control system is equipped with microcontrollers, and applies selected profiles to drivers of said three bridge rectifiers of said at least two transformers. 10- The method of claim 9, wherein said at least power transformers are a Y & Δ transformers and said bridge rectifiers are thyristor-based rectifiers; and wherein said harmonic reduction method further comprises the step of controlling all of said thyristors via a DC current control unit, where it conducts any one of said thyristors at any required time. 11- The method of claim 10, wherein said DC current control unit further comprises: a main control unit; a multi-profile programmer; a profile selector; a total coarse control unit; and individual fine control unit. 12- The method of claim 11, wherein said main control unit, receives all input parameters of V_(ac), I_(ac), V_(dc), I_(dc), desired data and other control signals; and sends a current control signal as an output to said total coarse control unit; wherein said multi-profile programmer generates available profiles for said system and wherein said profile selector, selects one of said available profiles (selected profile), based on said output coarse control signal of said total coarse unit; wherein said selected profile controls said drivers of said modular set, where said drivers submit trigger pulses to any of intended said multiple thyristors; wherein said output coarse control signal controls currents of all of said loads in said modular set. 13- The method of claim 12, wherein wherein said main control unit further sends another signal to said individual fine control unit, wherein its output signal (fine control output signal) controls said currents of any of said loads separately via said drivers. 14- The method of claim 13, wherein said harmonics are reduced and consequently overheating of said power transformer is prevented. 